diff --git a/src/collaboration/awareness.ts b/src/collaboration/awareness.ts
index 0444244d4..e859e3c70 100644
--- a/src/collaboration/awareness.ts
+++ b/src/collaboration/awareness.ts
@@ -214,6 +214,15 @@ export function renderCollaborationSelection(
 
 /** Select black or white text using the WCAG relative-luminance threshold. */
 export function contrastingTextColor(hexColor: string): '#000000' | '#ffffff' {
+  if (
+    typeof hexColor !== 'string' ||
+    !CURSOR_COLOR_PATTERN.test(hexColor)
+  ) {
+    throw new RangeError(
+      'collaboration contrast color must be a six-digit hexadecimal color',
+    );
+  }
+
   const red = Number.parseInt(hexColor.slice(1, 3), 16) / 255;
   const green = Number.parseInt(hexColor.slice(3, 5), 16) / 255;
   const blue = Number.parseInt(hexColor.slice(5, 7), 16) / 255;
diff --git a/src/collaboration/awarenessContrastColor.test.ts b/src/collaboration/awarenessContrastColor.test.ts
new file mode 100644
index 000000000..c1d6a1f2f
--- /dev/null
+++ b/src/collaboration/awarenessContrastColor.test.ts
@@ -0,0 +1,29 @@
+import { describe, expect, it } from 'vitest';
+import { contrastingTextColor } from './awareness.js';
+
+const INVALID_CONTRAST_COLOR_ERROR = new RangeError(
+  'collaboration contrast color must be a six-digit hexadecimal color',
+);
+
+describe('public collaboration contrast-color contract', () => {
+  it.each(['#fff', '#zzzzzz', 'red', ''])(
+    'rejects malformed color token %j instead of returning a plausible contrast',
+    (color) => {
+      expect(() => contrastingTextColor(color)).toThrowError(
+        INVALID_CONTRAST_COLOR_ERROR,
+      );
+    },
+  );
+
+  it('rejects non-string runtime input without coercion', () => {
+    expect(() => contrastingTextColor(7 as never)).toThrowError(
+      INVALID_CONTRAST_COLOR_ERROR,
+    );
+  });
+
+  it('preserves valid uppercase and lowercase six-digit colors', () => {
+    expect(contrastingTextColor('#FFFFFF')).toBe('#000000');
+    expect(contrastingTextColor('#000000')).toBe('#ffffff');
+    expect(contrastingTextColor('#777777')).toBe('#000000');
+  });
+});